This invention relates to color television receivers, monitors and other similar color television display systems, and more particularly to improving color resolution.
In a color television camera, three or four camera tubes are synchronized, each of three with separate one of red, green, and blue filters. In a three-tube camera, the red, green, and blue signals from the three primary color camera tubes are combined to produce a luminance (white) signal, designated Y. In a four-tube camera, one camera tube tube is used without a filter to generate the luminance signal directly. The color signals, are then encoded using a color subcarrier to produce two quadrature signals designated I and Q. These three signals, I, Q and Y, are then combined with synchronizing signals in a composite video signal modulated onto a carrier. A burst of the color carrier used in the color encoding process is modulated onto the carrier during line scan blanking periods so that it can be received and used as a "color burst" to synchronize a local oscillator, the output of which is then used as a color reference in decoding the I and Q signals into the primary color signals applied to the display unit with the luminance signal Y for reproducing the video image.
Since the bandwidths of the luminance and chrominance signals are markedly distinct, different low-pass filters are required: for the I signal, 1.3 MHz bandwidth; and for the Q signal, 0.5 MHz bandwidth; while the Y luminance signal having an 8.0 MHz bandwidth is generally filtered to about 4.2 MHz. As a consequence, delays are required at the encoder to bring the three signals into relative phase with each other prior to modulation on the carrier at the transmitter. Because the lower bandwidth signal encounters the greatest filter delay, the I signal must be delayed typically 0.2 .mu.s, and the Y signal must be delayed typically 1.2 .mu.s to compensate. Similar compensating delays must be used in the receiver.
While compensating delays will align the chrominance and luminance signals at the color television display system, the signal pulses which make up the image in color do not have the same rise and fall time. It is well known that the greater the frequency bandwidth the shorter the rise and fall time. For example, a perfect square wave signal requires an infinite number of harmonics of the fundamental frequency. As the bandwidth is limited more and more, the rise and fall times increase, and the square wave degenerates towards a sinusoidal waveform of the fundamental frequency. For example, the luminance signal will have a higher frequency response to scanning a red picket fence in full color than the chrominance signals, i.e., shorter rise and fall times at the edges of the picket fence slats will cause miscoloring of the leading and lagging edges due to saturation losses in the rise and fall time of the color signals. This results in deteriorated full color definition of the slats at their edges. It should, of course, be realized that because of the finite diameter of the scanning beam at the camera, even the luminance signal will not be a square waveform, but will instead have a waveform that is deteriorated toward the fundamental, but in phase with the ideal square waveform. This effect is known as "aperture distortion." Nevertheless, it would be desirable to effectively increase the frequency response (rise and fall times) of the color signals to that of the luminance signal for a more crisp display of the edges of images, thereby greatly enhancing the horizontal resolution of color television.